C++ N-last added items container - c++

Question

I try to find optimal data structure for next simple task: class which keeps N last added item values in built-in container. If object obtain N+1 item it should be added at the end of the container and first item should be removed from it. It like a simple queue, but class should have a method GetAverage, and other methods which must have access to every item. Unfortunately, std::queue doesn't have methods begin and end for this purpose.
It's a part of simple class interface:
class StatItem final
{
static int ITEMS_LIMIT;
public:
StatItem() = default;
~StatItem() = default;
void Reset();
void Insert(int val);
int GetAverage() const;
private:
std::queue<int> _items;
};
And part of desired implementation:
void StatItem::Reset()
{
std::queue<int> empty;
std::swap(_items, empty);
}
void StatItem::Insert(int val)
{
_items.push(val);
if (_items.size() == ITEMS_LIMIT)
{
_items.pop();
}
}
int StatItem::GetAverage() const
{
const size_t itemCount{ _items.size() };
if (itemCount == 0) {
return 0;
}
const int sum = std::accumulate(_items.begin(), _items.end(), 0); // Error. std::queue doesn't have this methods
return sum / itemCount;
}
Any ideas?
I'm not sure about std::deque. Does it work effective and should I use it for this task or something different?
P.S.: ITEMS_LIMIT in my case about 100-500 items

Answer 1

The data structure you're looking for is a circular buffer. There is an implementation in the Boost library, however in this situation since it doesn't seem you need to remove items you can easily implement one using a std::vector or std::array.
You will need to keep track of the number of elements in the vector so far so that you can average correctly until you reach the element limit, and also the current insertion index which should just wrap when you reach that limit.
Using an array or vector will allow you to benefit from having a fixed element limit, as the elements will be stored in a single block of memory (good for fast memory access), and with both data structures you can make space for all elements you need on construction.
If you choose to use a std::vector, make sure to use the 'fill' constructor (http://www.cplusplus.com/reference/vector/vector/vector/), which will allow you to create the right number of elements from the beginning and avoid any extra allocations.